Today, hard disk drives are widely used to store digital data because of their high storage capacity, economy, and random access capability. A hard disk drive is comprised of a stack of one or more magnetic disks. One or more transducers, also known as "heads," are used to write data onto the surfaces of these disks. Often, the same head is used to both write the data to and read the data from the disk. When data are written, the disk is spun about a spindle. As the head is moved radially across the surface of the spinning disk, data are written onto a number of concentric circles, commonly referred to as "tracks."
The digital data are stored as a series of binary bits. An encoding scheme is implemented in order to map these binary bits into a pattern that is compatible with the recording media of the data disk. In other words, the encoding scheme enables the hard disk drive system to physically distinguish between each of the bits stored in a track. One popular encoding scheme is a 1,7 code. The 1,7 code specifies that there should be a minimum of one and a maximum of seven "0's" between each pair of "1's". Hence, the spatial frequency of this type of encoding depends upon the particular sequence of digital data written onto the disk.
However, due to the far fringing of heads at low frequencies, the inductive heads sense long wavelength signals for wider than their physical width. For example, given a head width of approximately 6.3 microm, the head is still receptive to 0.1 MHz data that is four times the head width away. Indeed, low frequency signals exhibit a much wider reaching amplitude sensitivity than higher frequency signals. And since the 1,7 code has a spectral frequency content which extends all the way to 0 Hz, whenever an adjacent track is written, the head will inherently pick up that track's low frequency components. This adjacent track read problem might result in errors in reading data by traditional data qualifiers.
FIG. 1 shows a typical prior art read channel employing a traditional data qualification design. The data stored on the disk are converted into an analog electrical signal by the head. This electrical signal is amplified by a pre-amp. It is then AC coupled (high pass). Note that the frequency of this high pass is typically ##EQU1## the lowest data frequency. Further amplification and/or attenuation is performed by the automatic gain control circuitry 101. The electrical signal is then filtered by a low pass filter and the high frequency components are equalized by block 102. The output from the filter/equalizer 102 is input to the full wave rectifier 106. Furthermore, the peaks of each of the pulses of the electrical signal read from the disk are qualified according to a positive and a negative threshold of threshold qualifier block 103. The output of the threshold qualifier 103 is coupled to the input of latch 109. When the analog read signal exceeds either the positive or negative threshold, a qualifier pulse is generated. Also, the analog signal is differentiated by the differentiator block 104. The resulting differentiated signal is then input to the zero-crossing detector 105. Zero-crossing detector 105 is usually a zero volt comparator. Whenever the threshold qualifier signal is "high" and the zero-crossings signal changes state, a valid data pulse is output from latch 109. Note that when the differentiated signal is near a zero crossing, the zero-crossing signal toggles, but these spurious transitions are not qualified by the threshold and are, hence, ignored. The analog read-signal from filter 102 is also fed back through a full wave rectifier 106, peak detector 107, and AGC controller 108 to the AGC amplifier 101.
Thus, it can be seen that the typical prior art magnetic recording data qualifier passes the analog read signals through a simple low pass filter with perhaps some equalization boost. Unfortunately, this approach allows the low frequencies above the frequency of the A.C. coupling high pass to pass through. It has been determined that there can be approximately a 10% low frequency modulation on the data qualifier signal which causes errors. The adjacent track read problem is so severe that writing a 2T-2T-8T stress pattern (this stress pattern repetition rate emphasizes a low frequency corresponding to a 24T period, wherein T is the encoded dock period) on an adjacent track will often result in an error rate which causes the drive to fail. This problem is becoming even more acute as track widths become smaller and areal densities continue to increase.
Furthermore, it can be seen that the prior art of channel qualifiers typically has a threshold qualification path which is not differentiated. It is rather difficult to equalize and optimize the same head signal shapes for the qualifier path and for the differentiator path at the same time because of their conflicting requirements.
Thus, there is a need in the prior art of hard disk drives for a more robust threshold and qualification design. It would be preferable if such a design could differentiate the analog data signal first, thereby attenuating the low frequency adjacent track signals. It would also be highly preferable if such a design could improve the optimization capability of the read/write channel characteristics by eliminating the separate filtering of the threshold qualifier path.